Why Can't I Add an AjaxToolkit SliderExtender to My User Control, and How Can I Fix the 'Controls Collection Cannot Be Modified' Error?

Troubleshooting AjaxToolkit SliderExtender in User Controls

Adding an AjaxToolkit SliderExtender to a user control can sometimes result in the error: "The Controls collection cannot be modified because the control contains code blocks." This typically happens when server-side code blocks exist within the user control's markup.

Resolution:

The solution involves refactoring your server-side code. Instead of using Response.Write code blocks, implement data binding expressions. For example, replace Response.Write() code with equivalent data binding. This prevents the code from being interpreted as server-side code during the control's lifecycle, allowing modification of the Controls collection.

Here's a simplified illustration: (Note: The provided tag example in the original is incomplete and lacks context. A more relevant example would show a Response.Write statement within the user control's markup.)

Assume you have a Response.Write statement like this within your user control's .ascx file:

<% Response.Write(someVariable); %>

Replace it with a data binding expression:

<%= someVariable %>

Finally, add the following code to the code-behind file (.aspx.cs) of your master page:

protected void Page_Load(object sender, EventArgs e) {
  Page.Header.DataBind();
}

This DataBind() call ensures that data binding expressions are properly evaluated before the Controls collection is modified, thereby resolving the error and enabling the successful addition of the SliderExtender.

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